1. Field of the Invention
The present invention relates generally to a method for operating a Hybrid Automatic Repeat Request (HARQ) scheme in a broadband wireless access communication system and, more particularly, to a method for operating an uplink/downlink transmit response for effective utilization of a HARQ scheme.
2. Description of the Related Art
In a 4th generation (4G) communication system, which is the next generation communication system, research has been actively pursued to provide users with services having various qualities of service (QoS) at a high transmission speed of 100 Mbps. The current third generation (3G) communication system supports a transmit speed of about 384 kbps in an outdoor environment having relatively bad channel conditions and a transmit speed of a maximal 2 Mbps in an indoor environment having relatively good channel conditions
A wireless Local Area Network (LAN) communication system and a wireless Metropolitan Area Network (MAN) communication system generally support transmission speeds of 20 to 50 Mbps. Because the wireless MAN communication system has wide service coverage and supports a high transmission speed, it is suitable for supporting a high speed communication service. However, the wireless MAN system does not accommodate the mobility of a user, i.e., a subscriber station (SS), nor does it perform a handover according to the high speed movement of the SS. The wireless MAN system is a broadband wireless access communication system having a wider service area and supporting a higher transmission speed than the wireless LAN system.
Accordingly, in a current 4G communication system, a new type of communication system ensuring mobility and QoS for the wireless LAN system and the wireless MAN system supporting relatively high transmission speeds is currently being developed to support a high speed service to be provided by the 4G communication system. In this context, many studies are being conducted on using an Orthogonal Frequency Division Multiplexing (OFDM) scheme for high-speed data transmission over wired/wireless channels in the 4G mobile communication system. The OFDM scheme, which transmits data using multiple carriers, is a special type of a Multiple Carrier Modulation (MCM) scheme in which a serial symbol sequence is converted into parallel symbol sequences and the parallel symbol sequences are modulated with a plurality of mutually orthogonal subcarriers (or subcarrier channels) before being transmitted.
The Orthogonal Frequency Division Multiple Access (OFDMA) scheme is a Multiple Access scheme based on the OFDM scheme. In the OFDMA scheme, subcarriers in one OFDM symbol are distributed to a plurality of users (or SSs). Communication systems using the OFDMA scheme include an Institute of Electrical and Electronics Engineers (IEEE) 802.16a communication system and an IEEE 802.16e communication system. The IEEE 802.16 communication systems utilize the OFDM/OFDMA scheme in order to support a broadband transmit network for a physical channel of the wireless MAN system. Further, the IEEE 802.16 communication systems are broadband wireless access communication systems using a Time Division Duplex (TDD)-OFDMA scheme. Therefore, in the IEEE 802.16 communication systems, because the OFDM/OFDMA scheme is applied to the wireless MAN system, a physical channel signal can be transmitted using a plurality of sub-carriers, thereby achieving data transmission of high speed and high quality.
The OFDMA scheme can be defined by a two-dimensional access scheme, which is a combination of the Time Division Access (TDA) technology and Frequency Division Access (FDA) technology. Therefore, in data transmission using the OFDMA scheme, each OFDMA symbol is distributed to sub-carriers and transmitted through predetermined sub-channels. Herein, the sub-channel is a channel including a plurality of sub-carriers. In a communication system using the OFDMA scheme (OFDMA communication system), predetermined number of sub-carriers according to system conditions are included in one sub-channel.
FIG. 1 schematically illustrates a frame structure of a conventional TDD-OFDMA communication system. Referring to FIG. 1, the frame used in the TDD-OFDMA communication system is divided between downlink (DL) 149 and uplink (UL) 153, according to the time unit. In the frame, a protection time interval named ‘Transmit/receive Transition Gap (TTG) 151’ is arranged at a time interval for transition from the downlink 149 to the uplink 153 and a protection time interval named ‘Receive/transmit Transition Gap (RTG) 155’ is arranged at a time interval for transition from the uplink 153 to the next downlink. In FIG. 1, the horizontal axis represents the OFDM symbol number 145 of the OFDMA symbols and the vertical axis represents the sub-channel logical number 147 of the multiple sub-channels.
As illustrated in FIG. 1, one OFDMA frame includes a plurality of OFDMA symbols (for example, 12 OFDMA symbols). Also, one OFDMA symbol includes a plurality of sub-channels (for example, L sub-channels).
In the IEEE 802.16 communication system described above, all sub-carriers (especially, data sub-carriers) are distributed to all frequency bands, in order to obtain the frequency diversity gain. Further, in the IEEE 802.16 communication system, during the transmit/receive time interval, ranging is performed in order to adjust time offset and frequency offset, and adjust the transmit power.
Referring to the downlink 149, a preamble 111 for synch acquisition is located at the k-th OFDMA symbol, and broadcast data information such as a Frame Control Header (FCH) 113, a downlink MAP (DL-MAP) 115, and an uplink MAP (UL-MAP) 117, which must be broadcast to the subscriber stations, is located at the (k+1)-th or (k+2)-th OFDMA symbol. The FCH 113 includes two sub-channels to transfer basic information about the sub-channel, the ranging and the modulation scheme, etc. Downlink bursts (DL bursts) 121, 123, 125, 127, and 129 are located at the OFDMA symbols from the (k+2)-th OFDMA symbol to the (k+8)-th OFDMA symbol, except for the UL-MAP located at the (k+2)-th OFDMA symbol.
Referring to the uplink 153, preambles 131, 133, and 135 are located at the (k+9)-th OFDMA symbol and uplink bursts (UL bursts) 137, 139, and 141 are located at the OFDMA symbols from the (k+10)-th OFDMA symbol to the (k+12)-th OFDMA symbol. Further, a ranging sub-channel 143 is located at the OFDMA symbols from the (k+9)-th OFDMA symbol to the (k+12)-th OFDMA symbol.
In the IEEE 802.16 communication system, the transition from the downlink to the uplink is performed during the TTG 151. Further, the transition from the uplink to the downlink is performed during the RTG 155. Further, after the TTG 151 and the RTG 155, separate preamble fields 111, 131, 133, and 135 may be allocated to acquire synch between the transmitter and the receiver.
According to the frame structure of the IEEE 802.16 communication system, the downlink frame 149 includes a preamble field 111, an FCH field 113, a DL-MAP field 115, UL-MAP fields 117 and 119, and a plurality of DL burst fields (including a DL burst #1 field 123, a DL burst #2 field 125, a DL burst #3 field 121, a DL burst #4 field 127, and a DL burst #5 field 129).
The preamble field 111 is a field for transmitting a preamble sequence, which is a synch signal for synch acquisition for the transmit/receive time interval. Further, the FCH field 113 includes two sub-channels to transfer basic information about the sub-channel, the ranging and modulation scheme, etc. The DL-MAP field 115 is a field for transmitting the DL-MAP message. The UL-MAP fields 117 and 119 are fields for transmitting the UL-MAP messages. Here, the DL-MAP message includes Information Elements (IEs) as shown in Table 1 below.
TABLE 1SyntexSizeNotesDL-MAP_IE( ) { DIUC 4 bits if(DIUC==15) {  Extended DIUC dependentvariableSee 802.16a/16e OFDMA PHY Specifications  IE } else {  if(INC_CID==1) {The DL-MAP starts with INC_CID = 0. INC_CID istoggled between 0 and 1 by the CID_SWITCH_IE( ) (See 802.16a/16e OFDMA PHY Specifications)  N_CID 8 bitsNumber of CIDs assigned for this IE   for(n=0;n<N_CID;n++) {    CID16 bits   }  }  OFDMA Symbol Offset10 bits  Subchannel Offset 5 bits  Boosting 3 bits000: normal (not boosted)001: +6 dB010: −6 dB011: +9 dB100: +3 dB101: −3 dB110: −9 dB111: −12 dB  No. OFDMA Symbols 9 bits  No. Subchannels 5 bits }}
As shown in Table 1, a DIUC (Downlink Interval Usage Code) represents the object of a currently transmitted message and the modulation scheme in which the currently transmitted message is modulated before being transmitted. A CID (connection Identifier) represents the CID of each subscriber station corresponding to the DIUC.
OFDMA Symbol Offset represents the offset of a symbol resource allocated to each DL burst. Subchannel Offset represents the offset of a sub-channel resource allocated to each DL burst. Boosting represents a power value increased in the transmit power. ‘No. OFDMA Symbols’ represents the number of allocated OFDMA symbols. ‘No. Subchannels’ represents the number of allocated sub-channels.
As noted from Table 1, the downlink information of the IEEE 802.16 communication system is expressed in combination with information about each subscriber station according to the DIUC. Therefore, each subscriber station can analyze the data targeting the subscriber station itself, only after demodulating the entire DL-MAP message.
The UL-MAP message includes Information Elements (IEs) as shown in Table 2 below.
TABLE 2SyntexSizeNotesUL-MAP_IE( ) { CID16 bits UIUC 4 bits if(UIUC==12) {   OFDMA Symbol Offset10 bits   Subchannel Offset 6 bits   No. OFDMA Symbols 8 bits   No. Subchannels 5 bits   Ranging Method 3 bits000: Initial Ranging over two symbols001: Initial Ranging over four symbols010: BW Request/Periodic Ranging over one sysbol011: BW Request/Periodic Ranging over threesymbols100~111: reserved } else if(UIUC==14) {   CDMA_Allocation_IE ( )52 bits } else if(DIUC==15) {   Extended DIUC dependentvariableSee 802.16a/16e OFDMA PHY Specifications   IE } else {   OFDMA Symbol Offset10 bits   Subchannel Offset 5 bits   No. OFDMA Symbols 9 bits   No. Subchannels 5 bits   Mini-subchannel index 3 bits000: no mini-subchannels used001: starting with mini-subchannel 1010: starting with mini-subchannel 2011: starting with mini-subchannel 3100: starting with mini-subchannel 4101: starting with mini-subchannel 5110, 111: reserved }}
As shown in Table 2, a CID (connection Identifier) represents the CID of each corresponding subscriber station and an UIUC (Uplink Interval Usage Code) represents the object of the message to be transmitted by the corresponding subscriber station and the modulation scheme in which the message is modulated before being transmitted. The other information elements are similar to those in Table 1, so description of them will be omitted here.
According to the frame structure of the IEEE 802.16 communication system as described above, the uplink frame 153 includes a ranging sub-channel field 143, a plurality of preamble fields 131, 133, and 135, and a plurality of UL burst fields (a UL burst #1 field 137, a UL burst #2 field 139, and a UL burst #3 field 141).
The ranging sub-channel field 143 is a field for transmitting ranging sub-channels for the ranging, and the preamble fields 131, 133, and 135 are fields for transmitting preamble sequences, i.e. synch signals for synch acquisition for the transmit/receive time interval.
According to the prior art as described above, each subscriber station (SS) cannot be identified by the bursts transmitted from the base station (BS) to the SS but can be identified by the bursts transmitted from the SS to the BS. Accordingly, the prior art described above is not proper for use of a Hybrid Automatic Repeat Request (HARQ) scheme in order to increase the transmission throughput when high speed transmission is required in a digital communication system. Therefore, in the prior art, transmission efficiency may be degraded due to errors in the wireless data transmission.
Further, the IEs, as described above, must be transmitted to all SSs through the MAP message by the most robust modulation scheme, such that they can be delivered to an entire cell area covered by the BS. However, as noted from the above discussion, the IEs are inefficiently included in the MAP message, that is, control data of an over burdensome size in the high speed data transmission system must be maintained. Such inefficient control data decreases the proportion of the actual data traffic in the entire traffic.